COMPLETE DOCUMENT (1862 kb) - OECD Nuclear Energy Agency

The number of recycling steps in an LWR is limited due to the build-up of plutonium isotopes
that are not fissionable in a thermal neutron spectrum. Therefore devices producing large fluxes of fast
neutrons (fast reactors, accelerator driven sub-critical systems) remain necessary to incinerate the
degraded plutonium resulting from plutonium recycling in LWRs. Fuel management calculations have
shown that such a device can be operated in symbiosis with a park of LWRs. In Europe, the CAPRA
project has been defined to design a fast reactor which will consume as much plutonium as possible. In
the current CAPRA reference design, a mixed oxide core is foreseen with MOX fuel containing up to
45% plutonium oxide. However, this fuel does not dissolve in nitric acid solution as used in the
conventional PUREX process and alternative fuels are therefore being considered. The present focus in
Japan but also in Europe is on nitride fuels, since PuN easily dissolves in HNO 3 , the aqueous medium in
the PUREX process. This option requires the use of fully enriched 15 N reagents for PuN production in
order to avoid 14 C formation.
Presently, reprocessed uranium (REPU) is not being significantly recycled as a result of the
low price of natural uranium and the fact that it contains some 236 U which is a neutron poison and
decreases the reactivity of reprocessed uranium. Delay or absence of recycling of REPU in reactors will
lead to the build-up of the very radiotoxic decay products of 232 U and 234 U particularly 228 Th and 208 Tl.
Both stocks of REPU and of the depleted uranium from enrichment will need to be taken into account in
overall strategies for radioactive waste management. In a very long-term perspective, the total
radioactivity of depleted uranium if considered as waste material exceeds that of neptunium.
2.1.2 Minor actinides
If the recycling of plutonium can be achieved effectively on an industrial scale, the recycling
of americium should be considered next because of the following reasons:
• americium has the second highest contribution to the radiotoxicity in spent fuel;
• in the performance assessment of underground repositories, americium dominates the
radiotoxicity during the first 1 000 years;
• 241 Am is the precursor of long-lived 237 Np which generally dominates the normal evolution
scenarios in the performance assessment because of its long half-life;
• plutonium recycling increases the Am production.
As for plutonium and uranium, the most favourable transmutation reaction for the minor
actinides is fission since capture or (n, 2n) reactions generally produce other long-lived actinides. The
fission of the minor actinides can best be achieved in a fast-neutron flux in which most actinide isotopes
are fissionable. Even in this case, capture followed by fission is still an important process. In thermal
spectra, extra neutrons are required to convert the non-fissile into fissile isotopes (e.g. 241 Am into
242m Am) requiring extra fuel enrichment.
It is not surprising that the strategies for minor actinide transmutation in fast flux devices are
similar to those for Pu transmutation: they can either be mixed homogeneously in MOX fuel to yield the
so-called MINOX fuel, or loaded in special fuel assemblies as inert-matrix fuels, based on oxides or
possibly on nitrides and carbides. From a reactor physics point of view, both options are feasible.
However, limitations are set by the fuel fabrication.
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